The invention relates to radio-frequency filters which, due to their construction, have multiple simultaneous operating frequencies.
Filters based on transmission line resonators are fundamental components in modern radio apparatuses. Categorized according to the frequency response, the commonest filter types are band rejection and band pass filters which are used to attenuate high-frequency signals on a desired frequency band (band rejection) or outside a certain frequency band (band pass). In addition, low pass and high pass filters are used. Transmission line resonators, the resonating frequencies of which determine a filter's frequency response, are usually cylindrical coil conductors, or helixes, plated grooves or holes formed in a dielectric medium, coaxial outer/inner conductor pairs or strip lines formed on a board-like substrate. There are usually from two to about eight resonators in a filter. A filter is connected to the rest of the radio apparatus via input, output and control signal ports.
Mobile and cordless telephones are the most important application field of portable radio technology. In different parts of the world there are cellular telephone systems in use that differ from each other significantly in their operating frequency ranges. Of digital cellular telephone systems, the operating frequencies of the Global System for Mobile Telecommunications (GSM) are 890-960 MHz, those of the Japanese Digital Cellular (JDC) system in the 800 and 1500 MHz bands, of the Personal Communication Network (PCN), 1710-1880 MHz, and of the Personal Communication System (PCS), 1850-1990 MHz. The operating frequencies of the American AMPS mobile phone system are 824-894 MHz and those of the European cordless telephone system, DECT, 1880-1900 MHz.
With the mobility of people and communication between people increasing, there is a growing need for general-purpose phones that operate in different networks according to network availability and/or service prices. In dual mode radio telecommunications, the GSM and DECT (Digital European Cordless Telephone), or GSM and PCN (Personal Communication Network) or other systems can operate as pairs. The dual mode capability is also taken into account in the so-called third generation cellular systems (Universal Mobile Telecommunication System, UMTS/Future Public Land Mobile Telecommunications System, FLPMTS).
In a radio apparatus operating at two frequencies the filtering arrangement can be realized in two ways. In the first solution, the filters must meet the same requirements at both frequencies. The band pass filter must have a pass band at the both operating frequencies of the system, the band rejection filter must have corresponding stop bands and so forth. In the second solution, radio signals of different frequencies are directed via different routes, in which case the apparatus has got two parallel filters for each filtering function. The first solution is more advantageous in apparatuses where minimization of physical size is important.
In the design of shared filters, the choice of resonating frequencies for the transmission line resonators has proven problematic. The system operating frequencies listed above show that if the operating frequency of the first system (the one having the lower operating frequency of the two) is f0, the frequency of the second system for a dual mode phone is typically in the range from 1.5*f0 to 2.5*f0. A constant-impedance .lambda./4 transmission line resonator with a fundamental resonating frequency of f0 has odd harmonic resonating frequencies (fs1, fs2, . . . ) at the odd multiples of the fundamental resonating frequency. FIG. 1 shows a 2-circuit band pass filter implemented with constant-impedance .lambda./4 transmission line resonators Ra an Rb. FIG. 2 shows a typical frequency response for the filter. The filter's first pass band is at the frequency f0 and the next pass band, determined by the resonators' first odd harmonic resonating frequency fs1, is at the frequency 3*f0. The harmonic frequency is too high to be used for dual band/dual mode filtering.